Action Potentials in Neurons

Introduction to Action Potentials

Action potentials are rapid, transient changes in membrane potential that serve as the fundamental electrical signals in neurons. They enable the long-distance transmission of information along axons, ultimately leading to neurotransmitter release at synapses.

• Definition: An action potential (AP) is a stereotyped, all-or-none depolarization of the neuronal membrane, typically from a resting potential (~ -65 mV) up to about +40 mV.

• Initiation: APs are initiated at the axon hillock and propagate along the axon.

• Function: The primary function of an AP is the rapid, long-distance transmission of information within the nervous system.

Properties of Action Potentials

Key Characteristics

• Constant Amplitude and Duration: APs have a consistent amplitude (~100 mV) and duration (2–3 ms) in a given neuron type.

• Always Depolarizing: APs are always depolarizing events (membrane potential becomes less negative).

• All-or-None Principle: APs occur only if the membrane depolarization reaches a threshold (typically ~10 mV above rest). Subthreshold stimuli do not elicit APs.

• Threshold: The minimum depolarization required to trigger an AP.

• Refractory Periods: APs have two refractory periods:

  • Absolute Refractory Period: A phase during which no new AP can be generated, regardless of stimulus strength. Occurs during the falling phase of the AP.

  • Relative Refractory Period: A phase during which a stronger-than-normal stimulus is required to generate another AP. Occurs during the undershoot phase.

• Propagation Without Decrement: APs travel along the axon without decreasing in amplitude.

Table: Comparison of Absolute and Relative Refractory Periods

Action Potential Waveform and Phases

Phases of the Action Potential

• Resting State: Membrane at resting potential (~ -65 mV).

• Depolarization: Rapid influx of Na+ ions through voltage-gated Na+ channels causes the membrane potential to become positive.

• Repolarization: Na+ channels inactivate, and voltage-gated K+ channels open, allowing K+ efflux and return toward resting potential.

• Hyperpolarization (Undershoot): Membrane potential temporarily becomes more negative than resting due to continued K+ efflux.

• Return to Rest: K+ channels close, and the membrane returns to resting potential.

Mechanisms of Action Potential Generation

Voltage-Gated Ion Channels

• Voltage-Gated Na+ Channels: Open rapidly in response to depolarization (~10 mV above rest), allowing Na+ influx. Inactivate after ~1 ms.

• Voltage-Gated K+ Channels: Open more slowly (after ~1 ms delay), allowing K+ efflux, which repolarizes the membrane.

Table: Ion Concentrations and Equilibrium Potentials

Forces Influencing Ion Movement

• Diffusion Force: Ions move from high to low concentration.

• Electrostatic Force: Ions move according to electrical gradients.

• Driving Force: The net force (combination of diffusion and electrostatic) that determines ion movement.

Equations

• Nernst Equation: Determines the equilibrium potential for a given ion:

Patch-Clamp Technique

Experimental Evidence

• Patch-Clamping: A technique developed by Sakmann and Neher (Nobel Prize 1991) to measure currents through individual ion channels.

• Findings:

  • Na+ channels open rapidly (latency in microseconds) and inactivate even if depolarization continues.

  • K+ channels open with a delay (~1 ms) and do not inactivate during depolarization.

Conduction Velocity of Action Potentials

Determinants of Conduction Velocity

• Axon Diameter: Larger diameter axons have lower internal resistance, increasing conduction velocity.

• Membrane Resistance (Rm): Increased by myelination, which reduces ion leakage and increases speed.

• Maximum Velocity: Up to 100 meters/second in myelinated vertebrate axons; much slower in most neurons.

Table: Strategies to Increase Conduction Velocity

Myelination and Its Effects

Role of Myelin

• Myelin Sheath: Formed by oligodendrocytes (CNS) or Schwann cells (PNS), wraps around axons to increase membrane resistance.

• Effect: Reduces leakage of positive ions, allowing the action potential to travel faster and farther.

Summary Table: Key Properties of Action Potentials

Additional info:

• Action potentials are essential for neural coding, but their stereotyped nature means that information is encoded in the frequency of APs, not their shape or size.

• Myelination is a key evolutionary adaptation for rapid signal transmission in vertebrates.